This invention relates to complementary switching circuits such as used in electric motor control circuits to switch current flow in either direction through a set of motor field windings. Such complementary switching circuits are commonly used in computerized numerically controlled machine tools for energizing the field windings of A.C. induction motors with D.C. pulses of alternate polarity which, taken together, form an A.C. excitation voltage. In such inverter circuits, electronic switch elements such as transistors are connected in complementary pairs to the ends of the motor field windings, each transistor having its emitter-collector circuit coupled in series with one end of the corresponding field winding and one terminal of the D.C. voltage source while the other transistor of the pair has its emitter-collector circuit coupled in series with the same end of the corresponding field winding and the other terminal of the D.C. voltage source.
Normally, the transistors of each pair are turned on and off in complement, i.e., when one transistor of each pair is turned on the other is turned off, and vice versa. However, it is possible for a pair of transistors to "crossfire," i.e., to conduct simultaneously, due to a circuit malfunction such as: (1) a component failure in the driver circuit causing incorrect biasing of one transistor; (2) transient electrical noise at the logical input of the driver circuit or upstream from it; or (3) component or logical failure upstream from the driver circuit. "Crossfiring" is a serious malfunction since it shorts out the D.C. voltage source through the two crossfired transistors and burns out one or both transistors, thereby disabling the motor.